1. Field of the Invention
This invention relates to a reception apparatus, a reception method and a program.
2. Description of the Related Art
As one of modulation methods for a ground wave digital broadcast, an orthogonal frequency division multiplexing (OFDM) method has been proposed wherein a large number of orthogonal carriers are used and modulated by phase shift keying (PSK) or quadrature amplitude modulation (QAM).
The OFDM method has a characteristic that, since an entire transmission band is divided by a large number of subcarriers, the bandwidth per one subcarrier is narrow and, although the transmission speed per one subcarrier is low, the total transmission speed is equal to that of existing modulation methods.
The OFDM has another characteristic that, since a large number of subcarriers are transmitted in parallel, the symbol rate is low. Therefore, the time length of multipaths relative to the time length of one symbol can be reduced. Consequently, the OFDM method has a further characteristic that the influence of multipaths can be reduced.
Further, the OFDM has a still further characteristic that, since data are allocated to a plurality of subcarriers, a transmission circuit can be configured using an Inverse Fast Fourier Transform (IFFT) mathematical operation circuit which carries out Inverse Fourier Transform upon modulation and a reception circuit can be configured using a Fast Fourier Transform (FFT) mathematical operation circuit which carries out Fourier transform upon demodulation.
From such characteristics as described above, the OFDM method is frequently applied to a ground wave digital broadcast which is influenced significantly by a multipath disturbance. As standards for a ground wave digital broadcast which adopt the OFDM method, for example, DVB-T (Digital Video Broadcasting-Terrestrial), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) and ISDB-TSB (Integrated Services Digital Broadcasting-Terrestrial for Sound Broadcasting) are available.
FIG. 1 illustrates an OFDM symbol.
According to the OFDM method, transmission of a signal is carried out in a unit called OFDM symbol.
Referring to FIG. 1, one OFDM symbol is composed of an effective symbol which has a signal interval for which IFFT is carried out upon transmission, and a guard interval in which a waveform of part of the effective symbol is copied. The guard interval is inserted into a position before the effective symbol on the time axis.
By insertion of the guard interval, the OFDM method can prevent interference between OFDM symbols which occurs in a multipath environment.
A plurality of such OFDM symbols are gathered to form one OFDM transmission frame. For example, according to the ISDB-T standards, one OFDM transmission frame is formed from 204 OFDM symbols. An insertion position of a pilot signal is determined with reference to a unit of an OFDM transmission frame.
An OFDM method wherein a QAM type modulation method is used as a modulation method for subcarriers is influenced by multipaths and so forth upon transmission. Consequently, for each subcarrier, the amplitude and the phase upon reception become different from those upon transmission. Therefore, it is necessary for the reception side to carry out equalization of a signal so that the amplitude and the phase of a reception signal may be same as those of a transmission signal.
In the OFDM method, the transmission side discretely inserts a pilot signal of a predetermined amplitude and a predetermined phase into transmission symbols. The reception side determines a frequency characteristic of a transmission line based on the amplitude and the phase of the pilot signal and equalizes the reception signal based on the determined characteristic of the transmission line.
The pilot signal used for calculation of a transmission line characteristic in this manner is referred to as scattered pilot signal (hereinafter referred to as SP signal). FIG. 2 illustrates an arrangement pattern of the SP signal in OFDM symbols adopted by the DVB-T standards or the ISDB-T standards.
FIG. 3 is a block diagram showing an example of a configuration of an existing OFDM receiver.
Referring to FIG. 3, the OFDM receiver 100 shown includes a reception antenna 1, a tuner 2, an analog/digital (A/D) conversion circuit 3, an orthogonal demodulation circuit 4, a carrier production circuit 5, an FFT circuit 6, and an FFT interval control circuit 7. The OFDM receiver 100 further includes a transmission line distortion compensation circuit 8, an error correction circuit 9, a delay profile estimation circuit 10 and a frequency interpolation filter selection circuit 11.
The tuner 2 frequency converts an RF signal received by the reception antenna 1 into an IF signal and outputs the IF signal to the A/D conversion circuit 3.
The A/D conversion circuit 3 carries out A/D conversion for the IF signal supplied thereto from the tuner 2 and outputs a resulting digital IF signal to the orthogonal demodulation circuit 4.
The orthogonal demodulation circuit 4 carries out orthogonal demodulation using a carrier supplied thereto from the carrier production circuit 5 to acquire an OFDM signal of a base band from the IF signal supplied from the A/D conversion circuit 3, and outputs the acquired OFDM signal. The base band OFDM signal is a signal in the time domain before FFT mathematical operation is carried out.
In the following description, the OFDM signal of the base band before FFT mathematical operation is carried out is referred to as OFDM time domain signal. The OFDM time domain signal is a complex signal including a real axis component (I channel signal) and an imaginary axis component (Q channel signal) as a result of orthogonal demodulation. The OFDM time domain signal outputted from the orthogonal demodulation circuit 4 is supplied to the carrier production circuit 5, FFT circuit 6, FFT interval control circuit 7 and delay profile estimation circuit 10.
The carrier production circuit 5 produces a carrier of a predetermined frequency synchronized with the reception signal based on the OFDM time domain signal supplied from the orthogonal demodulation circuit 4 and outputs the produced carrier to the orthogonal demodulation circuit 4.
The FFT circuit 6 removes a signal within a range of the guard interval from a signal of one OFDM symbol based on an FFT trigger pulse supplied thereto from the FFT interval control circuit 7 to extract a signal within the range of the effective symbol length.
Further, the FFT circuit 6 carries out FFT mathematical operation for the extracted OFDM time domain signal to extract data orthogonally modulated in subcarriers. In particular, the start position of the FFT mathematical operation by the FFT circuit 6 is a position within a range from a position A in FIG. 1 which is the boundary of the OFDM symbol to another position B which is the boundary position between the guard interval and the effective symbol. The FFT mathematical operation range is called FFT interval, and the start position of the FFT interval is designated by the FFT trigger pulse supplied from the FFT interval control circuit 7.
The FFT circuit 6 outputs the OFDM signal representative of the extracted data. The OFDM signal is a signal in the frequency domain after the FFT mathematical operation is carried out. An OFDM signal after FFT mathematical operation is hereinafter referred to as OFDM frequency domain signal. The OFDM frequency domain signal outputted from the FFT circuit 6 is supplied to an SP extraction circuit 8-1 and a division circuit 8-4 of the transmission line distortion compensation circuit 8.
The FFT interval control circuit 7 determines an FFT interval based on the OFDM time domain signal supplied from the orthogonal demodulation circuit 4 and a delay profile estimated by the delay profile estimation circuit 10. Then, the FFT interval control circuit 7 outputs an FFT trigger pulse which designates the start position of the determined FFT interval to the FFT circuit 6.
The transmission line distortion compensation circuit 8 includes an SP extraction circuit 8-1, a time direction transmission line characteristic estimation circuit 8-2, a frequency interpolation circuit 8-3 and a division circuit 8-4.
The SP extraction circuit 8-1 extracts an SP signal from the OFDM frequency domain signal supplied thereto from the FFT circuit 6 and removes a modulation component of the SP signal to estimate a transmission characteristic of a subcarrier at the arrangement position of the SP signal. The SP extraction circuit 8-1 outputs a signal representative of the estimated transmission line characteristic to the time direction transmission line characteristic estimation circuit 8-2.
The time direction transmission line characteristic estimation circuit 8-2 estimates a transmission characteristic of the subcarrier, at which the SP signal is arranged, at the position of each of the OFDM symbols juxtaposed in the time direction, that is, in the OFDM symbol direction, based on the transmission characteristic estimated by the SP extraction circuit 8-1. In FIG. 2, the vertical direction is the time direction, and the horizontal direction is the frequency direction.
For example, the time direction transmission line characteristic estimation circuit 8-2 uses a transmission line characteristic at the position of an SP signal SP1 and another transmission line characteristic at the position of another SP signal SP2 of FIG. 2 estimated by the SP extraction circuit 8-1 to estimate a transmission line characteristic of a subcarrier at the position of a different symbol in a region A1 of FIG. 2.
Since an SP signal is inserted for every 12 subcarriers at the same time point as seen in FIG. 2, the time direction transmission line characteristic estimation circuit 8-2 estimates a transmission line characteristic of a subcarrier at the position of an OFDM symbol for every three subcarriers. The time direction transmission line characteristic estimation circuit 8-2 outputs a signal representative of the estimated transmission line characteristic for every three subcarriers. The signal outputted from the time direction transmission line characteristic estimation circuit 8-2 is supplied to the frequency interpolation circuit 8-3 and the delay profile estimation circuit 10.
The frequency interpolation circuit 8-3 carries out an interpolation process for interpolating a transmission characteristic in the frequency direction to estimate a transmission characteristic of the subcarrier at the position of each OFDM symbol in the frequency direction from the transmission line characteristics for every three subcarriers supplied from the time direction transmission line characteristic estimation circuit 8-2. The frequency interpolation circuit 8-3 includes a plurality of interpolation filters having different filter bands and carries out an interpolation process using the interpolation filters.
For example, the frequency interpolation circuit 8-3 estimates the transmission characteristic of a subcarrier at the position of an OFDM symbol for which estimation of the transmission characteristic is not carried out as yet from among the positions of OFDM symbols included in a region A2 of FIG. 2. The estimation of the transmission characteristic is carried out using transmission characteristics estimated already by the SP extraction circuit 8-1 and the time direction transmission line characteristic estimation circuit 8-2.
As a result, the transmission line characteristics of all subcarriers at the positions of the OFDM symbols are estimated. The frequency interpolation circuit 8-3 outputs a signal obtained by execution of the interpolation process using the interpolation filter of the filter band designated by the filter selection signal supplied from the frequency interpolation filter selection circuit 11 as a signal representative of an estimation result of the transmission line characteristic to the division circuit 8-4.
The division circuit 8-4 divides a component of the signal representative of the transmission characteristics of all subcarriers supplied from the frequency interpolation circuit 8-3 from the OFDM frequency domain signal supplied from the FFT circuit 6 to remove a component of distortion by the transmission line from the OFDM frequency domain signal. The division circuit 8-4 outputs the OFDM frequency domain signal from which the distortion component is removed to the error correction circuit 9.
The error correction circuit 9 carries out a deinterleave process for a signal interleaved by the transmission side and further carries out such processes as depuncture, Viterbi decoding, spread signal removal and RS (Reed Solomon) decoding. The error correction circuit 9 outputs data obtained by the processes as decoded data to a circuit at a succeeding stage.
The delay profile estimation circuit 10 determines the time response characteristic of the transmission line to estimate a delay profile of the transmission line. For example, the delay profile estimation circuit 10 carries out IFFT for the transmission line characteristic estimated by the time direction transmission line characteristic estimation circuit 8-2 to estimate a delay profile. The transmission line characteristic estimated by the time direction transmission line characteristic estimation circuit 8-2 is a frequency characteristic, and a time response characteristic obtained by carrying out IFFT for the transmission line characteristic is a delay profile.
A signal representative of the delay profile estimated by the delay profile estimation circuit 10 is supplied to the FFT interval control circuit 7 and the frequency interpolation filter selection circuit 11. It is to be noted that, as a method for the delay profile estimation, also a method of utilizing a matched filter (MF) whose tap coefficient is the guard interval period to estimate a delay profile from an OFDM time domain signal is known.
The frequency interpolation filter selection circuit 11 determines a delay spread based on the delay profile, that is, the position of a path on the time axis, estimated by the delay profile estimation circuit 10, and selects a filter band corresponding to the delay spread from among filter bands of the interpolation filters provided for the frequency interpolation circuit 8-3. The frequency interpolation filter selection circuit 11 outputs a filter selection signal which designates the selected filter band to the frequency interpolation circuit 8-3.
FIG. 4 shows an example of a configuration of the frequency interpolation circuit 8-3.
Referring to FIG. 4, the frequency interpolation circuit 8-3 includes frequency interpolation filter circuits 8-3a0 to 8-3aN-1 and a selector circuit 8-3b. A signal representative of the transmission characteristic for every three subcarriers outputted from the time direction transmission line characteristic estimation circuit 8-2 is inputted to a corresponding one of the 8-3a0 to 8-3aN-1, and a filter selection signal outputted from the frequency interpolation filter selection circuit 11 is inputted to the selector circuit 8-3b. 
The frequency interpolation filter circuits 8-3a0 to 8-3aN-1 carry out an interpolation process using the interpolation filters provided thereto and output signals representative of a result of the interpolation to the selector circuit 8-3b. In the example of FIG. 4, the frequency interpolation filter circuit 8-3a0 carries out an interpolation process using the interpolation filter for the filter band BW0, and the frequency interpolation filter circuit 8-3a1 carries out an interpolation process using the interpolation filter for the filter band BW1. The frequency interpolation filter circuit 8-3aN-1 carries out an interpolation process using the interpolation filter for the filter band BW(N-1). FIG. 5 illustrates the filter bands BW0 to BW3 on the time axis.
In the example of FIG. 5, the filter band BW0 has the greatest bandwidth and the filter band BW3 has the smallest bandwidth. The position of an upwardly directed void triangle represents the position of the center of the filter band. The interpolation process is carried out such that the center position of the filter band becomes same as the center position of the delay spread.
The selector circuit 8-3b selects a signal obtained by carrying out the interpolation process using the interpolation filter of the filter band designated by the filter selection signal from among the signals supplied from the frequency interpolation filter circuits 8-3a0 to 8-3aN-1 and outputs the selected signal to the division circuit 8-4.
Japanese Patent Laid-Open No. 2006-311385 discloses a technique for detecting the quality of a signal after equalization and controlling an optimum filter coefficient from among a plurality of filter coefficients in response to the detected quality.